Among integrated circuit (IC) fabrication techniques, lithography process is an important technique used in forming selected circuit patterns on a substrate such as a semiconductor wafer. As integration density of IC is increasing in fabricating smaller and more complex circuits of semiconductor devices, advanced lithography techniques are accordingly required in producing smaller critical dimensions (CDs) and fine patterns.
In advanced lithography techniques, different reticles (also referred to as masks or photomasks) are utilized for various requirements of the lithography process. For example, binary intensity masks (BIMs) have good performance and are cost effective because of its simplest structure with transparent portions and totally opaque portions. In addition, phase-shift masks (PSMs) have relatively opaque portions and have been used to overcome problems associated with light diffraction and to enhance the lithography exposure resolution. Besides, chromeless phase-shift masks (CPMs) do not have opaque portions and reduce diffraction effects by combining both phase-shifted light and non-phase-shifted light so that constructive and destructive interference takes place, and therefore improve resolution and depth of focus of a projected image of an optical system. Therefore, each kind of the reticles has its own structure and advantage in different scopes of application.
Because requirement in lithography processes keeps increasing, forming smaller CDs and fine patterns becomes even challenging. Therefore, reticles utilized in lithography processes have to become more delicate to overcome various difficulties in different scopes of application. As such, improvements in reticles and methods of fabricating thereof continue to be sought.